Typically a sifting screen is composed of a first woven cloth of stainless steel wires stretched within or across a frame and supported by a second woven cloth of stainless steel wires also stretched within or across the frame, the mesh pitch of the second woven cloth being much coarser than that of the first woven cloth.
Since the second woven cloth is intended to prevent the first one from sagging and to assist in de-binding of the top cloth, particularly when loaded with material to be sifted, wire of considerably greater cross-section is normally employed for the second cloth.
The failure of the wires of woven wire sifting screens can be attributed to two factors. The first type of wire failure is commonly referred to as fatigue and appears as breaks in the wires in high flexure regions of the woven cloth. Continued use of a screen after such breaks have occurred rapidly results in adjoining wires breaking at similar points along their length causing cracks in the cloth which widen and elongate until they appear as relatively large elongate openings or slits. These render the screen useless for continued filtration of particulate material since the latter can readily pass through these openings instead of remaining on the upper surface of the fine mesh screen.
The second type of wire failure only appears in screens which are constructed from two or more woven wire cloths stretched one over the other and in which one of the cloths is constructed from fine guage wire with a close weave spacing and the other (usually the lower one as described) is constructed from a heavier guage wire having a relatively coarse weave spacing. In such an arrangement, the knuckles at the interstices of the warp and weft of the heavier guage cloth constitute high points on which the closely woven wire mesh of the upper cloth are repetitively impaled as one mesh vibrates against the other in use. The finer guage wire tends to wear away and fracture in the region of these interstices causing so-called "pin-holing" of the closely woven cloth.
Although techniques have been proposed to deal with the problem, the solutions which have been put forward so far have been to do with the repair of damaged screens rather than prevention of the problem in the first place. Thus it has been proposed to construct a screen from separate sub-frames to which the various cloths are attached and which are removable to allow damaged regions to be replaced with fresh cloth, to effect "a running repair", thereby obviating the need to replace the whole of the screen assembly merely because a pin-hole has appeared in one localised region of the screen.
Whilst techniques which speed up and facilitate the repair of in-situ screens have their place, the condition of a screen after use typically when filtering water or oil based muds from drilling rigs, is such that a considerable amount of time has to be spent cleaning the screen to allow it to be inspected and damaged regions found and replaced by new inserts. Any downtime of a machine such as this when associated with a process such as sea bed drilling for oil and gas, is not only costly but because sometimes weather conditions and other factors limit the time available for drilling etc, such a failure at a crucial point in time can be critical to the success of the overall operation.
It is therefore an object of the present invention to provide an alternative form of sifting screen construction which will have a longer operational life than that of types hitherto and which under ordinary operating conditions should have a predictable ordinary life span allowing replacement to be performed at set periods of time much in the same way as other components are replaced at regular servicing intervals.
References herein to "wire" are not intended to mean exclusively metal wire but also wire of non metallic materials including plastics, carbon fibre and KEVLAR.TM..